Proposal: Taxonomic Studies of the Dichanthelium acuminatum (Poaceae) complex

Graduate Student Investigator: Rick L. Hammer (Texas A&M University)

Introduction

Dichanthelium acuminatum is a grass species (Poaceae:Panicoideae:Paniceae) that consists of approximately 10 varieties or subspecies, depending upon the taxonomic treatment consulted. It is probably the most polymorphic and taxonomically troublesome species of the genus. Freckmann and Lelong (2003), in their recent Flora of North America treatment, delimit 10 subspecies and note that their treatment attempts to define the major variants present but “does not fully reflect the intricate reticulate pattern of morphological variation that exists^.” In an earlier treatment of the genus, Gould and Clark (1978) recognized eight D. acuminatum varieties, some of which are not in alignment with the Freckmann and Lelong (2003) treatment. There has been no prior research that has examined the infraspecific classification of D. acuminatum using either a phenetic approach employing morphological characters or a population genetic approach using molecular markers. The goal of the research outlined in this proposal is to approach these infraspecific taxonomic difficulties from a population level using both phenetic and molecular markers. Field work wil be conducted to samle natural populations of most of the subspecies. Herbarium material will be obtained to supplement the field data and to extend the geographic sampling of morphological data.

Goals

Determine levels of morphological and genetic divergence within and between populations of each of the subspecies. This will provide data for testing subspecies boundaries via population genetic analysis of molecular data and phenetic analysis of morphological data but will also reveal patterns of geographic distribution of genetic diversity in a given subspecies. Subspecies with geographically variable genetic diversity may be more likely to have greater levels of morphological variability across their range. This might indicate the need for a more regional approach to constructing keys in taxonomic treatments.

Test subspecific boundaries in D. acuminatum complex by analysis of SSR data. Data analysis using standard population genetic analysis methods and metrics.

Test subspecific boundaries in D. acuminatum complex by phenetic analysis of morphological data. Data analysis using standard phenetic analyses such as principal components analysis of selected morphological characters.

Evaluate the current taxonomy. Look for possible realignments maybe on a regional basis (initially).

Background

Dichanthelium acuminatum is common to much of North America with the range of the taxa extending into northern South America. All subspecies of the D.. acuminatum complex are diploid (2n=18) (Gould and Clark 1978). The most recent treatment of the complex is that of Freckmann and Lelong (2003) in their treatment of the genus Dichanthelium for the Flora of North America. Here they recognize 10 subspecies: spretum, longiligulatum, lindheimeri, sericeum, thermale, acuminatum, columbianum, fasixculatum, implicatum, and leucothrix. An earlier comprehensive treatment for the complex is that of Gould and Clark (1978) in which they recognize eight varieties: villosum, acuminatum, thurowii, implicatum, wrightianum, densiflorum, lindheimeri, and longiligulatum. Among the differences in the two treatments are Freckmann and Lelong’s (2003) recognition of subsp. thermale and sericeum both of which were subsumed in var. acuminatum in Gould and Clark’s treatment. The synonomy of D. acuminatum is burdensome as many authors have proposed numerous realignments of the subspecific taxa, including shifts of some varieties to the species level and vice versa (Gould and Clark 1978; Freckmann 1981; Lelong 1984; Freckmann and Lelong 2003). For example, Gould and Clark (1978) placed 46 names in synonomy in their treatment of D. acuminatum. The difficult and confusing synonomy is a result of extensive morphological variation found among members of the complex. Spellenberg (1975) studied western U.S. populations of some of the subspecies and proposed that autogamy and hybridization are a common means that account for much of the morphological variation and thus the taxonomic difficulty encountered in the complex. The genus itself has been variously treated as a subgenus of Panicum but recent molecular data reinforce the morphological arguments for treating Dichanthelium as a distinct genus (Guissani et al. 2001).

Experimental Methods

Plant collection. Field collections will begin in spring of 2004 and will sample populations in Texas, Louisiana and Oklahoma as most of the D. acuminatum subspecific taxa occur in these states. Field sampling in spring 2005 will include populations beyond TX. LA, OK. Monies awarded on the basis of the current proposal to ASPT will be used to fund field collection activities.

Morphological examination of populations. Diagnostic characters will be measured from individual plants collected in the field and from herbarium specimens. Characters will be chosen based on prior taxonomic treatments.

Molecular genetic examination of populations. To include DNA extraction, polymerase chain reaction amplification and nuclear microsatellite assay. Protocols and procedures to be determined.

Data analysis. This will include univariate and multivariate analysis of morphological data and population-genetic metrics for molecular data to include measures of genetic polymorphisms, measures of genetic diversity and expected heterozygosity. Phylogenetic analysis will be performed to investigate genetic relationships among the populations.

Literature Cited

Freckmann, R.W. 1981. Realignments in the Dichanthelium acuminatum complex (Poaceae). Phytologia 99-110.

Freckmann, R.W., and M.G. Lelong. Dichanthelium. pp. 406-450. In Barkworth, M.E., K.M. Capels, S. Long, and M.B. Piep, eds. 2003. Magnoliophyta: Commelinidae (in part): Poaceae, part 2. Flora of North America North of Mexico, volume 25. Oxford University Press, New York.

Gould, F.W., and C.A. Clark. 1978. Dichanthelium (Poaceae) in the United States and Canada. Annals of the Missouri Botanical Garden. 65:1088-1132.

Guissani, L.M., J.H. Cota-Sanchez, F.O. Zuloaga, and E.A. Kellog. 2001. A molecular phylogeny of the grass subfamily Panicoideae (Poaceae) shows multiple origins of C4 photosynthesis. American Journal of Botany. 88:1993-2001.

Lelong, M.G. 1984. New combinations for Panicum subgenus Panicum and subgenus Dichanthelium (Poaceae) of the Southeastern United States. Brittonia 36: 262-273.

Spellenberg, R.W. 1975. Autogamy and hytbridization as evolutionary mechanisms in Panicum subgenus Dichanthelium (Gramineae). Brittonia 27: 87-95.